1. Field of the Invention
The present invention relates to a method for determining fluid influx or loss when drilling wells from a floating rig, for example a drill ship or a semi-submersible rig.
2. Description of the Related Art
In certain situations in the petroleum industry, oil bearing formations are to be found beneath the sea bed. Where the sea bed is up to 350 ft below the sea level, bottom supported drilling rigs such as jack-up rigs can be used. However, in deeper water it is not possible for the drilling rig to rest on the bottom and a floating platform must be used. Floating platforms such as drill ships or semi-submersible rigs can operate in much deeper water than bottom supported rigs but do suffer from problems in maintaining a steady positional relationship with the sea bed. While horizontal movements can be controlled to some degree by dynamic positioning systems and anchoring, vertical movement or "heave" due to wave action remains.
It is current practise to utilise a drilling fluid or mud in petroleum or geothermal well drilling. The mud is pumped into the drillstring at the surface and passes downwardly to the bit from where it is released into the borehole and returns to the surface in the annular space between the drillstring and borehole, carrying up cuttings from the bit back to the surface. The mud also serves other purposes such as the containment of formation fluids and support of the borehole itself. When drilling a well, there exists the danger of drilling into a formation containing abnormally high pressure fluids, especially gas, which may pass into the well displacing the mud. If this influx is not detected and controlled quickly enough, the high pressure fluid may flow freely into the well causing a blowout. Alternatively, some formations may allow fluid to flow from the well into the formation which can also be undesirable.
Fluid influx (or a "kick") or fluid loss (lost circulation) can be detected by comparing the flow rate of mud into the well with the flow rate of mud from the well, these two events being indicated by a surfeit or deficit of flow respectively. However, in floating rigs, heave motion effectively changes the volume of the flow path for mud flow to and from the well making the detection of kicks or lost circulation difficult in the short term.
A method and apparatus for detecting kicks and lost circulation is described in U.S. Pat. No. 3,760,891 in which the return mud flow is monitored and the values accumulated over overlapping periods of time. By comparing the flow from one period with that of a previous period and comparing with preselected values, the flow rate change is determined. However, this technique is relatively slow to determine anomalous flow situations.